1. Field of the Invention
The present disclosure relates to a processing liquid that can be used in an inkjet recording method of forming an image on a recording medium such as paper by discharging ink, which contains at least water and a pigment, onto such recording medium, and to an inkjet recording method that uses the processing liquid.
2. Description of the Related Art
Conventionally, known is an inkjet recording device that causes ink to adhere onto a recording surface of a recording medium such as PPC paper and forms an image by discharging ink, which contains at least water and a pigment, onto such recording medium from an ink discharge opening (nozzle) of an inkjet recording head based on image information that was sent, for example, from another device. Such an inkjet recording device generally comprises a conveyor belt for conveying a recording medium, and the inkjet recording head that is disposed facing the conveyor belt. An image is formed on the recording medium by discharging ink from a plurality of ink discharge openings aligned on the inkjet recording head onto the recording medium while conveying the recording medium on the moving conveyor belt. The recording medium to which ink has adhered and an image is formed is ultimately discharged outside of the inkjet recording device.
With plain paper such as PPC paper, and not a recording medium such as coated paper for use in photo printing or the like in which its recording surface is coated with resin, the pigment contained in the ink that is discharged to the recording medium excessively permeates to the inside of the recording medium. Consequently, there are a problem in that the image density of the image formed on the paper will deteriorate, a problem in that the blurring of the image will increase, and a problem in that the image will strike through. The term “strike through” refers to the phenomenon where the ink (pigment) discharged to the recording medium permeates to the reverse side of the recording medium, and the front-side image appears on the reverse side of the recording medium. This phenomenon will impair duplex printing.
In order to deal with these kinds of problems, for instance, known is a method of, prior to causing ink to adhere to the recording medium, causing to adhere in advance an aqueous solution of multivalent metal salt such as calcium salt or magnesium salt as the processing liquid to the recording medium. Consequently, aggregation of the pigment contained in the ink that was discharged to the recording medium is promoted by reacting with the multivalent metal ions created as a result of the multivalent metal salt dissolving in water on the recording surface, whereby the excessive permeation of the pigment into the recording medium is inhibited. Thus, it is possible to avoid the problem of the image density deteriorating, the problem of the image blurring, and the problem of the image striking through, and it is thereby possible to obtain a high quality image.
In order to improve the effect of the foregoing processing liquid; that is, in order to increase the pigment aggregation promotion effect, the multivalent metal salt content in the processing liquid should be increased. Nevertheless, if the multivalent metal salt content is increased in the processing liquid, there is a problem in that the multivalent metal salt easily precipitates due to the drying of the processing liquid; that is, due to the evaporation of the solvent of the processing liquid. When the multivalent metal salt precipitates, the pigment aggregation promotion effect of the processing liquid will diminish. In addition, if the multivalent metal salt content in the processing liquid increases, there is an additional problem in that the solubility of an organic additive will deteriorate. An organic additive is, for example, an additive as an organic compound such as a viscosity modifier or a surface active agent (as a wetting agent or the like) to be added to the ink or the processing liquid.